I have a Mimas with the DAC card. It replaced An Ayre integrated and a Codex DAC. I find the internal DAC card to be much more natural sounding then the Codex, as good as that DAC is. I can't say if it will sound better than the MW Oppo. I used to have a MW Logitech Transporter and I still think it was the best sounding Digital I have ever heard. I sold it when Logitech discontinued the Transporter and I was concerned about the ongoing support. Dan's Mods are excellent sounding though I've not heard the OPPO Mod.
I will be adding the phono stage next. At the moment I am using an external phone stage because my TT is 4 meters away from the amp and and I have no way to get it anywhere near the Mimas. I have demoed the phono stage and feel it’s great sounding device that bests my Nova II by a large margin. YMMV and I suggest that you try to hear it with your equipment to help you make the best decision You can check my profile to see my complete system. Good Luck!